In U.S. Pat. No. 4,440,871, the preparation of a number of phosphorus-containing molecular sieves is described.
The patent particularly describes processes for the manufacture of numerous crystalline microporous silicoaluminophosphates (SAPO's) including SAPO-34, employing sources of silicon (e.g. a silica sol), aluminium (e.g., hydrated aluminium oxide), and phosphorus (e.g., orthophosphoric acid), and an organic template, for example tetraethylammonium hydroxide (TEAOH), isopropylamine (iPrNH2) or di-n-propylamine (DPA). The patent, the disclosure of which is incorporated by reference herein, gives X-ray diffraction data for the SAPO's and describes their utilities in catalysis and absorption.
International Application WO 00/06493 describes obtaining phosphorus-containing molecular sieves of lower particle size and narrower size distribution by agitation, e.g., stirring or tumbling.
EP-A-541 915 is concerned with the conversion of methanol to olefins (MTO), especially light (C2 to C4) olefins, using an aluminophosphate crystalline molecular sieve catalyst. The specification describes the advantages of small particle size catalysts in MTO processes, and provides a process for facilitating the manufacture of a small particle size material by stirring the synthesis mixture, producing SAPO-34 of median particle diameters, expressed as a mass distribution, in the range of about 0.6 to 1.4 μm.
EP-A-185 525 describes a process in which SAPO-37 is manufactured using a two-phase synthesis mixture. In an example there are used an aqueous phase containing phosphoric acid, alumina, and tetraethyl and tetrapropyl ammonium hydroxides as organic templates, and an organic phase comprising tetraethyl orthosilicate in hexanol, a solvent immiscible with water.
International Application WO 01/36328 describes a process in which a SAPO-34 molecular sieve is manufactured in the form of isocrystalline spheroidal particles of from 0.5 to 30 μm diameter using an aqueous synthesis mixture comprising a template, sources of the elements essential to the structure of the sieve and an organic solvent miscible with water, the purpose of the solvent being to solubilize the source of the silicon in the aqueous synthesis mixture, and a surfactant as morphology modifying agent. The silicon source may be a tetraalkyl orthosilicate.
The spheroidal particles of SAPO-34 are said to have a textured surface formed by crystallites the width (largest dimension) of which is from about 0.05 to 2.5 μm, as determined by scanning election microscopy. Although it is stated that reducing the concentration of silicon in the synthesis mixture reduced the diameter of the spheroids, the product of the SAPO-34 examples is always spheroidal.
There remains a need, however, for a method whereby very small individual crystallites of SAPO-34 can be manufactured, as opposed to the clusters resulting from the process described in WO 01/36328.
It has now been found that by omitting the surfactant a SAPO-34 product in the form of individual crystallites of much reduced particle size may be obtained.